A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Such a system of illuminating a target and collecting data from the reflected radiation is often used to calculate the overlay error, OV for a pattern. This can be achieved by forming a plurality of superimposed gratings on the substrate and measuring the overlay error between the gratings. To measure the overlay in, for example, the X direction, a grating varying in the X direction, as shown in FIG. 9a of the accompanying drawings is used. To measure overlay in, for example, the Y direction, a grating varying in the Y direction, as shown in FIG. 9b of the accompanying drawings is used. Each of these targets occupies an area on the substrate that could otherwise be used for other patterns, such as those that form the basis for an integrated circuit, and thus occupies valuable space.
An alternative target pattern is shown in FIG. 10 of the accompanying drawings. In this target, the pattern varies in both the X and Y direction and therefore only one target is used. However, cross-talk between the X and Y directions occurs when this target is used, thus reducing the accuracy of the results and increasing the complexity of the computation of the results.